Airborne transmission of SARS-CoV-2 in indoor environments: A comprehensive review

The COVID-19 pandemic has caused millions of deaths and great economic loss globally. There has been substantial evidence supporting the airborne transmission of SARS-CoV-2. Airborne route has been considered as a major transmission pathway, which can spread the disease over a longer distance and time. The viral loads in the respiratory tract of a virus carrier are typically below 10⁹ RNA copies/mL and are related to the emission rate of pathogens. Most particles expelled during respiratory activities are smaller than 1–2 µm in diameter. Viral aerosols can remain infectious for hours under typical indoor conditions. Sunlight contributes greatly to the viability inactivation of SARS-CoV-2. The outbreaks in different scenarios are reviewed based on existing data. Most outbreaks were related to long-term care facilities, K-12 schools, restaurants, retail facilities, and offices. The Wells-Riley model for estimating the risk of airborne transmission is introduced, along with model parameters such as the quanta generation rate, virus-containing particle size distribution, and inactivation rate. The effectiveness of various IAQ control strategies for mitigating the airborne transmission risk is analyzed, including PPE, ventilation strategies, partitions, air cleaning, and disinfection technologies, and occupancy control strategies. Both benefits and costs should be considered in designing the control strategies.